Hey, I have the same problem as you. What i think the problem is that the battery is just plain crap. Me and my brother both have the same problem and i have messed with my filmware and he hasnt. So it most likely is the battery. Go in e-bay and look for ipod shuffel batterys that are 300mah and buy one of those and replace your battery if you have soldering skills. thats what im planing on doing
